Derivative Instruments and Hedging Activities - Fair Values of Commodity Derivatives Including Offsetting of Assets and Liabilities (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	$ (68)	$ (90)
Gross Amounts Offset in the Statement of Financial Position	38	47
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(30)	(43)
Net Amount	(30)	(43)
Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	78	86
Gross Amounts Offset in the Statement of Financial Position	(49)	(57)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	29	29
Net Amount	29	29
Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(146)	(176)
Gross Amounts Offset in the Statement of Financial Position	87	104
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(59)	(72)
Net Amount	(59)	(72)
CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(49)	(56)
Gross Amounts Offset in the Statement of Financial Position	24	29
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(25)	(27)
Net Amount	(25)	(27)
CECONY [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	22	27
Gross Amounts Offset in the Statement of Financial Position	(13)	(15)
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	9	12
Net Amount	9	12
CECONY [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts of Recognized Assets/(Liabilities)	(71)	(83)
Gross Amounts Offset in the Statement of Financial Position	37	44
Net Amounts of Assets/(Liabilities) Presented in the Statement of Financial Position	(34)	(39)
Net Amount	(34)	(39)
Financial instruments [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial instruments [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial instruments [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial instruments [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial instruments [Member] | CECONY [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Financial instruments [Member] | CECONY [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member] | CECONY [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member] | CECONY [Member] | Derivative assets [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position
Cash collateral received [Member] | CECONY [Member] | Derivative liabilities [Member]
Derivatives, Fair Value [Line Items]
Gross Amounts Not Offset in the Statement of Financial Position